Interesting. There are a few more details that do not appear to be covered in that description. In order for TTL to work, the external strobe needs to send out a preflash that the camera reads. Presumably this happens during the "sync pulse." The canon protocol also supports up to three different flash intensities - I would assume part of the sync pulse is an address bit that tells which strobe to send out the preflash.

Are there any commercial UW strobes that support this protocol? I was trying to get it to work with my Sea&Sea YS-D2 but it did not seem to respond correctly.

Designed by Pavel who is very active here (and is a great source of information). It works great with my 5D mk4 and my S&S YS-D1 strobes. I believe that Turtle also has a unit available now too, but you don't get Pavel's invaluable help with the Turtle.

Johnny, it looks like that solves a different problem from what pbvales was mentioning. That (and the turtle) translate from the canon hotshoe signals to optical TTL signals that the flashes can understand.

It sounded like pbvalves was talking about building a strobe that can understand the Canon specific optical wireless TTL signals (the kind that is used by on-camera flashes, or the ST-E2 transmitter).